An inkjet recording method is a method wherein fine droplets of ink are ejected by various working principles and deposited on a recording medium such as paper or film to perform image recording. The inkjet recording method is widely utilized in terminal printers, facsimiles, plotters, sheet printing, and the like for the reasons of low noise, easiness of multicolor recording of full color images and the like, possibility of performing high-speed recording, lower recording cost than that in other printing devices, and so forth.
With regard to the image formed by the inkjet recording method, by increasing its image resolution and extending its color reproducible range, it is possible to obtain a recorded matter comparable to a multicolor print obtained by a plate-making method and a printed image obtained by a color photographic method, so that demand for images formed by the inkjet printing method has been rapidly increasing recently in design usage where high color-developing ability and color reproducibility are required, such as posters, displays, leaflets, package proofs, and the like.
With the extension of the above demand, since it becomes necessary to eject a large amount of ink for attaining the high color-developing ability and color reproducibility in the multicolor printing and the like, it is desired to develop an inkjet recording sheet having a large ink-receiving capacity corresponding to the ejected amount.
In general, two kinds of inkjet recording sheets are known, i.e., a void-type one and a polymer resin-type one. The void-type recording sheet has an ink-receiving layer where pores are formed of a filler such as silica, alumina, or calcium carbonate on a substrate, wherein a solvent of ink is absorbed in the pores through capillary attraction, and thereby only a color material component as a color-developing component is fixed on the surface. On the other hand, the polymer resin-type recording sheet has an ink-receiving layer formed of a polymer resin on a substrate, wherein the polymer resin itself absorbs a solvent while being swollen, and thereby only a color material component as a color-developing component is fixed on the surface. Dryness of the color materials can be enhanced by separation of the solvent from the color materials and absorption thereof by the receiving layer. Therefore, the higher the absorbing ability of the ink-receiving layer is, the more the dryness of the color materials is enhanced.
However, conventional inkjet recording sheets are suitable for dye ink but are not necessarily suitable for pigment ink. The reason is considered to be that pigment particles contained in pigment ink are generally very large as compared with dye, and thus it is required to develop an inkjet recording sheet suitable for recording with pigment ink.
On the other hand, in the design usage, a high transparency is required for the ink-receiving layer on which an image is to be recorded. In general, the void-type ink-receiving layer formed of a filler such as silica, alumina, or calcium carbonate has a low transparency and hence the polymer resin-type ink-receiving layer is preferable from the viewpoint of transparency.
Heretofore, as such a highly transparent polymer resin-type ink-receiving layer-forming material, polyvinyl alcohol, polyvinylpyrrolidone, starch, and water soluble cellulose derivatives have been proposed. However, in the method, particularly, the absorbing ability of aqueous pigment ink is poor. Resulting from the fact, when an ejected amount of ink is increased for high color-developing ability and color reproducibility, there is a problem that aggregation (phenomenon of forming a lump of ink on the surface) and spreading of pigment ink occur at the time of ink drying or the image is cracked due to excessive swelling of the receiving layer during drying. On the other hand, when the ejected amount of ink is too small, the color-developing ability is not attained and also the color reproducibility is poor. These problems caused by the absorbing ability are remarkably observed particularly under a low-temperature/high-humidity environment. Moreover, when the absorbing ability is poor, the drying property of the pigment to be fixed on the surface is also poor.
In this connection, Patent Document 1 proposes a recording sheet having an image-receiving layer (ink-receiving layer) comprising a cationic acrylic silicone emulsion-based resin and a cationic urethane-based resin, as an inkjet recording sheet for aqueous pigment ink usage. In Patent Document 1, resulting from such a constitution, it is said that there is obtained an inkjet recording sheet which is excellent in transparency or glossiness and also excellent in absorbing ability, drying property, and color-developing ability of ink.
Moreover, Patent Document 2 proposes, as an inkjet recording sheet for aqueous pigment ink usage, a recording sheet: having an image-receiving layer comprising two kinds of resins, i.e., an aqueous urethane resin and an aqueous acrylic resin; and containing a water soluble urethane polymer having an oxazoline group and an acrylic water soluble self-emulsifying epoxy curing agent as crosslinking agents. In Patent Document 2, resulting from such a constitution, it is said that there is obtained an inkjet recording sheet which is excellent in transparency or glossiness, also excellent in absorbing ability, drying property, and color-developing ability of ink, and satisfactory in water resistance property.
On the other hand, there is required an inkjet recording sheet having a further high quality in addition to transparency and absorbing ability. As one of the required characteristic properties, there is prevention of excessive adhesion (blocking) of the sheets themselves which occurs at the time when a plurality of the inkjet recording sheets are superposed. Generally, in order to prevent the adhesion of the inkjet recording sheets themselves, it is conducted to form unevenness on the sheet surface by adding fine particles such as silica particles into the ink-receiving layer. However, there are problems that the absorbing ability decreases by the influence of addition of the fine particles which do not contribute to the absorption of ink and also the transparency decreases by the influence of light scattering induced by fine particles.
[Patent Document 1] JP-A-2006-88341
[Patent Document 2] JP-A-2005-74880